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COHN— STUDY OF THE 



[April 23, 



of the heart, and certain others with the contraction of the ven- 

 tricles, the lower pair of heart chambers. At first all records were 

 obtained by applying electrodes directly to the surface of the heart 

 as it lay exposed in the opened chest, but in 1889 Waller showed that 

 one could obtain records of these currents by applying suitably con- 

 structed electrodes to the surface of the body without opening or 

 injuring it. This discovery opened the way for studying these currents 

 in the human subject. Waller alsosbowed which were favorable and 

 which unfavorable locations for placing electrodes, by varying the sit- 

 uation at which they were applied. From a consideration of records ob- 



FiG. I. After Waller. To indicate the spread of the cardiac action 

 current through the human subject. 



tained from a number of positions, now called leads or derivations, he 

 determined the location of a plane, on the two sides of which the 

 greatest differences in potential were developed (Fig. i). Those 

 locations were called favorable which yielded curves showing the 

 largest waves, and the records were called electrocardiograms. The 

 differences between electrocardiograms taken indirectly in this way 

 and those taken directly from the surface of the heart are ones of 

 contour and do not involve the important time relations of the 

 various elements composing the curve. 



With the knowledge that the heart discharged action currents, 

 and a method for conducting these from the uninjured surface of 

 the body to a registering instrument, the time was ripe for the con- 

 struction of a galvanometer better fitted to the purposes of physio- 

 logical and medical research. W. Einthoven of Leyden in 1902-6 

 described and built this instrument. Its completion at that time was 



